TFT pixel threshold voltage compensation circuit with light-emitting device initialization

ABSTRACT

A pixel circuit for a display device includes a drive transistor configured to control an amount of current to a light-emitting device depending upon a voltage applied to a gate of the drive transistor; a second transistor connected to the gate of the drive transistor and a second terminal of the drive transistor such that, when the second transistor is in an on state the drive transistor becomes diode-connected such that the gate and a second terminal of the drive transistor are connected through the second transistor; a light-emitting device that is connected at a first node to the second terminal of the drive transistor and at a second node to a first voltage supply; a third transistor that is connected between an initialization voltage supply and the first node of the light-emitting device, wherein a node N 1  is a connection of the second terminal of the drive transistor, the first node of the light-emitting device, and the third transistor; and at least one capacitor having a first plate that is connected to the gate of the drive transistor and a second plate that is connectable to a reference voltage supply. The pixel circuit is operable in an initialization phase to initialize circuit voltages, in a compensation phase to compensate for variations in drive transistor properties, in a programming phase to program a greyscale value to the pixel circuit, and in an emission phase in which the light-emitting device emits light corresponding to the greyscale value.

TECHNICAL FIELD

The present invention relates to design and operation of electroniccircuits for delivering electrical current to an element in a displaydevice, such as for example to an organic light-emitting diode (OLED) inthe pixel of an active matrix OLED (AMOLED) display device.

BACKGROUND ART

Organic light-emitting diodes (OLED) generate light by re-combination ofelectrons and holes, and emit light when a bias is applied between theanode and cathode such that an electrical current passes between them.The brightness of the light is related to the amount of the current. Ifthere is no current, there will be no light emission, so OLED technologyis a type of technology capable of absolute blacks and achieving almost“infinite” contrast ratio between pixels when used in displayapplications.

Several approaches are taught in the prior art for pixel thin filmtransistor (TFT) circuits to deliver current to an element of a displaydevice, such as for example an organic light-emitting diode (OLED),through a drive transistor. In one example, an input signal, such as alow “SCAN” signal, is employed to switch transistors in the circuit topermit a data voltage, VDAT, to be stored at a storage capacitor duringa programming phase. When the SCAN signal is high and VDAT is isolatedfrom the circuit by a switch transistor closing, the VDAT voltage isretained by the capacitor and this voltage is applied to a gate of adrive transistor. With the drive transistor having a threshold voltageV_(TH), the amount of current to the OLED is related to the voltage onthe gate of the drive transistor by:

$I_{OLED} = {\frac{\beta}{2}\left( {V_{DAT} - V_{OLED} - V_{TH}} \right)^{2}}$(β is a constant related to the properties of the drive transistor).

TFT device characteristics, especially the TFT threshold voltage V_(TH),may vary, for example due to manufacturing processes or stress and agingof the TFT device during the operation. With the same VDAT voltage, theamount of current delivered by the drive TFT could vary by a largeamount due to such threshold voltage variations. Therefore, pixels in adisplay may not exhibit uniform brightness for a given VDAT value.

Conventionally, therefore, OLED pixel circuits have high toleranceranges to variations in threshold voltage and/or carrier mobility of thedrive transistor by employing circuits that compensate for mismatch inthe properties of the drive transistor. For example, an approach isdescribed in U.S. Pat. No. 7,414,599 (Chung et al., issued Aug. 19,2008), which describes a circuit in which the drive TFT is configured tobe a diode-connected device during a programming period, and a datavoltage is applied to the source of the drive transistor.

With such circuit configuration, however, the anode of the OLED is notreset in relation to the programming phase. Rather, there will beresidual voltage at the OLED anode. When emission starts and theemission current flows through the OLED during the emission phase, theOLED will need some time to refresh the data voltage at the anode. Afirst problem with this is that it may affect the true black state. Ifthe previous frame data voltage corresponds to a white grayscale and thecurrent frame data voltage corresponds to a black grayscale, forexample, there will be some light emission due to the residual voltageat the beginning of the emission phase. The true black state will becompromised. A second problem is memory effects from the previous framedata. If the programmed current is a low current, it could take asignificant time to refresh the anode to the programmed value. Duringthe refresh period, the light emission could vary due to the previousresidual data at the anode of the OLED, which means the same programmeddata could have different light emission as affected by the previousframe data, especially for relative small emission current.

Another approach is described in U.S. Pat. No. 8,314,788 (Choi, issuedNov. 12, 2012). In such circuit, the diode-connection voltage for adrive transistor is pulled down by changing the voltage level at the topplate of the storage capacitor. There are significant drawbacks withsuch configuration and method. First, when pulling down the gate voltageof the drive transistor, the diode-connected drive transistor is forwardbiased, and there could be a large instant current to the OLED. This maycause an instance of high luminance light, which would prevent a pixelfrom ever having a true black state. Second, the anode of the OLED andthe gate voltage of the drive transistor would hold the voltage from theprevious frame. As there is no initialization or reset scheme, thevoltage from the previous frame could affect the programmed voltage forthe current frame. Therefore, the current to the OLED during a frame maybe affected by the state in the previous frame, as well as by theapplied data.

Other approaches to address the above problems have proven deficient.U.S. Pat. No. 7,936,322 (Chung et al., issued May 3, 2011) describes ascheme to reduce the number of transistors to five by overlapping scanand emission control signals. This approach, however, could cause aleakage current during the programming phase, which may affect theblackness in low current operations. U.S. Pat. No. 8,237,637 (Chung,issued Aug. 7, 2012) describes a scheme to improve the blackness andremove the memory effects on the anode of the OLED by adding one moretransistor between the initial voltage and the anode. Thisconfiguration, however, increases the transistor number to seven in thecircuit, which will lower the yield and be difficult to implement inhigh resolution applications requiring a small geometry. U.S. Pat. No.8,314,788 (Choi, issued Nov. 12, 2012) describes a scheme to reduce thenumber of transistors by pulling down the gate voltage of the drivetransistor to save the isolation transistor between the drivingtransistor and OLED. With such configuration, however, there could behigh instant current during programming phase. There is noinitialization scheme, and thus memory effects could affect the trueblackness. U.S. Pat. No. 9,337,439 (Kwon, issued May 10, 2016) describesa scheme to improve the blackness by using previous data, but the numberof transistors is still high and the residual voltage at the anode ofthe OLED could still cause some light leakage for low emission current.U.S. Pat. No. 9,489,894B2 (Yin et al.) describes a scheme to use ELVDDas an initial signal, which reduces the signal lines by one. Thisapproach, however, still has the same number of transistors as U.S. Pat.No. 7,414,599, and the same residual memory effects at the anode of theOLED.

SUMMARY OF INVENTION

The present invention relates to pixel circuits that are capable ofcompensating the threshold voltage variations of the drive transistorwith fewer transistors than in conventional configurations, withadditionally (1) reducing or eliminating the possible memory effectsassociated with the OLED device and drive transistor from their statesin the previous frame; and (2) improving the capability of a pixel toemit very little or no light during an initialization phase andtherefore to have a true black state.

A pixel circuit for delivering a required current to an OLED has a drivetransistor that sets the current supplied to the OLED during an emissionphase according to a voltage at the gate of the drive transistor, thegate voltage being stored on a storage capacitor. The pixel circuit isconfigured such that during an initialization phase, an initializationvoltage (INIT) is applied simultaneously at the gate of the drivetransistor, at the first plate of the storage capacitor, and at a firstterminal (and preferably the anode) of the OLED.

In exemplary embodiments, the value of the initialization voltage is setsuch that the difference between the initialization voltage and thevoltage at the second terminal (preferably the cathode) of the OLED(ELVSS) is:

-   -   i) less than the threshold voltage of the OLED (VTH_(OLED)),        which is the minimum forward bias which when applied to the OLED        for the duration of the initialization phase causes light        emission discernible to a human eye: INIT−ELVSS<VTH_(OLED)    -   and    -   ii) greater than the reverse bias on the OLED which causes        damage to the OLED. For example: INIT−ELVSS>−2V.        The initialization voltage may be in the range:    -   −2V<INIT−ELVSS<VTH_(OLED).

In exemplary embodiments the value of INIT−ELVSS is less than 0V suchthat the OLED is reverse biased during the initialization phase.Advantageously, this enables the voltage range for the pixel circuit(i.e., the difference between the highest voltage in the pixel circuitand the lowest voltage in the pixel circuit) to be reduced while stillproviding effective compensation of the threshold voltage of the drivetransistor. The reduced range for the pixel circuit enables lower powerconsumption for a driver of the display.

The drain of the drive transistor may be connected to the anode of theOLED such that during the initialization phase, an initializationvoltage (INIT) is applied simultaneously at the gate of the drivetransistor, at one plate of the storage capacitor, at the anode of theOLED, and at the drain of the drive transistor. During thisinitialization phase, the memory effects associated with the OLED andthe gate of the drive transistor from their states in the previous frameare reduced or eliminated.

The pixel circuit may be further configured with one or more of thefollowing features and modes of operation:

-   -   (1) During a subsequent threshold voltage compensation phase, a        relatively high fixed voltage is applied at the source of the        drive transistor, and thereby a voltage related to the threshold        voltage of the drive transistor may be stored on the storage        capacitor. Preferably, this voltage is set at a level that does        not cause the OLED to emit light.    -   (2) During a subsequent data programming phase, a reference        voltage (VREF) is applied to the second plate of the storage        capacitor which pulls down the voltage at the gate of the drive        transistor. The gate voltage of the drive transistor may be        pulled down close to the initialization voltage. A data voltage        may then be programmed to the storage capacitor through a        diode-connected state of the drive transistor.    -   (3) VREF is set to boost the gate voltage of the drive        transistor to an emission voltage level that drives the light        emission from the OLED during an emission phase.    -   (4) At the beginning of the emission phase, an emission second        initialization phase is used to improve the threshold        compensation performance when the current passed by the drive        transistor during the subsequent emission phase is low. During        this phase a second initialization voltage is applied to the        anode of the OLED.

Embodiments of the present invention have advantages over conventionalconfigurations. Such advantages include, for example, using as little asonly five transistors and one storage capacitor, thereby providing asmaller circuit as compared to conventional configurations. In addition,the pixel circuit configurations of the present disclosure improve thetrue blackness by reversely biasing the OLED in phases that precede theemission phase, and improve the accuracy of threshold voltagecompensation of the drive transistor especially for low OLED currents.

An aspect of the invention is a pixel circuit for a display device thatis operable in an initialization phase, in a compensation phase, in aprogramming phase, and in an emission phase. In exemplary embodiments,the pixel circuit includes: a drive transistor configured to control anamount of current to a light-emitting device during the emission phasedepending upon a voltage applied to a gate of the drive transistor; asecond transistor connected to the gate of the drive transistor, whereinthe second transistor is in an on state during the initialization,compensation and programming phases and is in an off state during theemission phase, and when the second transistor is in the on state thedrive transistor becomes diode-connected such that the gate and a secondterminal of the drive transistor are connected through the secondtransistor; a light-emitting device that is connected at a first node tothe second terminal of the drive transistor and at a second node to afirst voltage supply; a third transistor that is connected to the firstnode of the light-emitting device, wherein a node N1 is a connection ofthe second terminal of the drive transistor, the first node of thelight-emitting device, and the third transistor; and at least onecapacitor having a first plate that is connected to the gate of thedrive transistor and a second plate that is connectable to a referencevoltage. The third transistor is in an on state during theinitialization phase to connect a first initialization voltage to thenode N1, and to the first plate of the capacitor through the secondtransistor, to initialize a voltage across the light-emitting device,and the third transistor is in an off state during the emission phase;and when the reference voltage is applied to the second plate of thecapacitor, a threshold voltage of the drive transistor is at leastpartially compensated during the compensation phase. The pixel circuitfurther is operable during an emission second initialization phase,wherein during the second initialization phase the third transistor isin an on state to connect a second initialization voltage to the nodeN1.

In exemplary embodiments, the pixel circuit further includes a fourthtransistor that is connected to a third terminal of the drivetransistor, wherein the fourth transistor is in an on state during theprogramming phase to apply a data voltage corresponding to a greyscalevalue for the light-emitting device for the emission phase. The pixelcircuit further includes a fifth transistor that is connected to thethird terminal of the drive transistor, wherein during the emissionphase the fifth transistor is in an on state to connect the drivetransistor to a second voltage supply. The pixel circuit further mayinclude a sixth transistor that is connected to the third terminal ofthe drive transistor, wherein during the compensation phase the sixthtransistor is in an on state to apply a fixed voltage to the thirdterminal of the drive transistor.

Another aspect of the invention is a method of operating a pixel circuitfor a display device including the steps of: providing a pixel circuitaccording to any of the embodiments; and performing an initializationphase, a compensation phase, a programming phase, and an emission phase.During the initialization phase, memory effects from previous frames arereduced by performing the steps of: placing the fourth transistor in theoff state, and placing the fifth transistor in the off state todisconnect the second voltage supply from the pixel circuit; placing thethird transistor in the on state to apply a first initialization voltageto the first node of the light-emitting device; placing the secondtransistor in an on state, wherein the drive transistor becomesdiode-connected through the second transistor; and placing the thirdtransistor in the off state at an end of the initialization phase.During the compensation phase, a threshold voltage of the drivetransistor is at least partially compensated by the steps of: placingthe fourth transistor in an on state and applying a reference datavoltage to the third terminal of the drive transistor, wherein thereference data voltage is set such that a voltage across thelight-emitting device is below a threshold voltage of the light emittingdevice. During the programming phase, a data voltage corresponding to acurrent greyscale value is applied by the steps of: applying a referencevoltage to the second plate of the capacitor to a level for programmingthe full data range; changing the data voltage supply from the referencedata voltage to a current data voltage corresponding to a requiredcurrent through the light-emitting device during the emission phase;placing the fourth transistor in an off state after applying the currentdata voltage; and placing the second transistor in an off state suchthat the drive transistor is no longer diode-connected, wherein thecurrent data voltage is stored by the storage capacitor. During theemission phase, control of light emission is performed by the steps of:adjusting the reference voltage applied to the second plate of thecapacitor to change the gate voltage of the drive transistor to theoperational voltage range, in which the drive transistor controls theamount of current to the light-emitting device; placing the fifthtransistor in the on state to connect the second power supply to thedrive transistor; and controlling an amount of current to thelight-emitting device depending upon a voltage applied to a gate of adrive transistor.

The operating method may include, after the programming phase and at thebeginning of the emission phase, performing an emission secondinitialization phase to reset the voltage at the first node of thelight-emitting device by the steps of: placing the third transistor inan on state and applying a second initialization voltage to the firstnode of the light-emitting device; and changing the state of the thirdtransistor from the on state to the off state prior to the emissionphase.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing depicting a first circuit configuration inaccordance with embodiments of the present invention.

FIG. 2 is a timing diagram associated with the circuit configuration ofFIG. 1.

FIG. 3 is a drawing depicting a second circuit configuration inaccordance with embodiments of the present invention.

FIG. 4 is a timing diagram associated with the circuit configuration ofFIG. 3.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described withreference to the drawings, wherein like reference numerals are used torefer to like elements throughout. It will be understood that thefigures are not necessarily to scale.

FIG. 1 is a drawing depicting a first circuit configuration 10 inaccordance with embodiments of the present invention, and FIG. 2 is atiming diagram associated with the circuit configuration of FIG. 1. Inthis example, the circuit 10 is configured as a TFT circuit thatincludes multiple p-type transistors T1-T5 and a single storagecapacitor Cst having a first plate that is connected to the gate of thedrive transistor T1. The circuit elements drive a light-emitting device,such as for example an OLED. The light-emitting device (OLED) has anassociated internal capacitance, which is represented in the circuitdiagram as C_(oled). In addition, although the embodiments are describedprincipally in connection with an OLED as the light-emitting device,comparable principles may be used with display technologies that employother types of light-emitting devices, including for example micro LEDsand quantum dot LEDs.

More specifically, FIG. 1 depicts the TFT circuit 10 configured withmultiple p-MOS or p-type TFTs. T1 is a drive transistor that is ananalogue TFT, and T2-T5 are digital switch TFTs. As referenced above,Cst is a capacitor, and C_(oled) is the internal capacitance of the OLEDdevice (i.e., C_(oled) is not a separate component, but is inherent tothe OLED). The OLED further is connected to a first power supply ELVSSas is conventional.

The OLED and the TFT circuit 10, including the transistors, capacitorsand connecting wires, may be fabricated using TFT fabrication processesconventional in the art. It will be appreciated that comparablefabrication processes may be employed to fabricate the TFT circuitsaccording to any of the embodiments.

For example, the TFT circuit 10 (and subsequent embodiments) may bedisposed on a substrate such as a glass, plastic, or metal substrate.Each TFT may comprise a gate electrode, a gate insulating layer, asemiconducting layer, a first electrode, and a second electrode. Thesemiconducting layer is disposed on the substrate. The gate insulatinglayer is disposed on the semiconducting layer, and the gate electrodemay be disposed on the insulating layer. The first electrode and secondelectrode may be disposed on the insulating layer and connected to thesemiconducting layer using vias. The first electrode and secondelectrode respectively may commonly be referred to as the “sourceelectrode” and “drain electrode” of the TFT. The capacitor may comprisea first electrode, an insulating layer and a second electrode, wherebythe insulating layer forms an insulating barrier between the first andsecond electrodes. Wiring between components in the circuit, and wiringused to introduce signals to the circuit (e.g. SCAN, EMI, VDATA, VREF)may comprise metal lines or a doped semiconductor material. For example,metal lines may be disposed between the substrate and the gate electrodeof a TFT, and connected to electrodes using vias. The semiconductorlayer may be deposited by chemical vapour deposition, and metal layersmay be deposited by a thermal evaporation technique.

The OLED device may be disposed over the TFT circuit. The OLED devicemay comprise a first electrode (e.g. anode of the OLED), which isconnected to transistors T1, T2, and T3 in this example, one or morelayers for injecting or transporting charge (e.g. holes) to an emissionlayer, an emission layer, one or more layers for injecting ortransporting electrical charge (e.g. electrons) to the emission layer,and a second electrode (e.g. cathode of the OLED), which is connected tofirst power supply ELVSS in this example. The injection layers,transport layers and emission layer may be organic materials, the firstand second electrodes may be metals, and all of these layers may bedeposited by a thermal evaporation technique.

Referring to the TFT circuit 10 in combination with the timing diagramin FIG. 2, the TFT circuit 10 operates to perform in the followingphases: (1) a first initialization phase, (2) a threshold voltagecompensation phase (3) a data programming phase, and (4) an emissionphase for light emission including an optional emission secondinitialization phase. The time period through the completion of the dataprogramming phase is referred to in the art as the “horizontal time” or“1H” as illustrated in FIG. 2 and subsequent the timing diagrams. Ashort 1H time is a requirement for displays with a large number ofpixels in a column, as is necessary for high-resolution displays. In theexample of FIG. 1, therefore, 1H encompasses the first initializationphase, the threshold voltage compensation phase, and the dataprogramming phase.

In this first embodiment, during the first initialization phase memoryeffects from residual voltages from the previous frame are essentiallyeliminated. At the outset, an EMI signal level is set to a high voltagevalue, causing transistor T5 to be off. Consequently, the source of thedrive transistor T1 is disconnected from a second power supply, VDD. Inaddition, an initialization voltage signal (INIT) is changed from a highreference voltage level, INIT_H, to a low reference voltage levelINIT_L. Preferably, INIT_H is similar to the voltage at the anode of theOLED that results in the lowest light emission from the OLED during theemission phase for typical operation of the display. Accordingly,INIT_L, being lower than INIT_H, has a value that may reverse bias theOLED, i.e., INIT_L may be is less than ELVSS. The value of INIT_L alsoshould be set so as not to cause significant degradation of the OLED.

Further during the first initialization phase, a SCAN1 signal level ischanged from a high value to a low value, causing transistor T3 to beturned on. Consequently, the low initialization voltage, INIT_L isapplied at node N1, which is at the anode of the OLED. The applicationof the initialization voltage INIT_L operates to eliminate memoryeffects from a previous frame, as any residual voltage at the anode ofthe OLED from the previous frame will be reset to INIT_L.

Further during the first initialization phase, a SCAN2 signal level ischanged from a high voltage value to a low voltage value, causingtransistor T2 to be turned on. Consequently, the drive transistor T1becomes “diode-connected” through transistor T2. Diode-connected refersto the drive transistor T1 being operated with its gate and a secondterminal (e.g., source or drain) being connected, such that currentflows in one direction. When the drive transistor T1 is diode-connectedin this fashion, the gate and drain of the drive transistor areinitialized to the INIT_L voltage through transistors T2 and T3. In thismanner, the initialization voltage INIT_L is applied simultaneously atthe gate of the drive transistor, at a first (top) plate of the storagecapacitor, at the anode of the OLED, and at the drain of the drivetransistor. During this initialization phase, therefore, the memoryeffects associated with the OLED and the gate of the drive transistorfrom their states in the previous frame are reduced or eliminated.

The turn-on sequence of the transistors, specifically turning on T3followed by T2, is a preferred sequence of operation to ensure thatthere is no light emission from the OLED. For example, if T2 is turnedon before T3, a residual voltage at the gate of the drive transistor T1could cause light emission in the diode-connected state when the gate ofdrive transistor T1 is connected to the anode of the OLED throughtransistor T2. As referenced above, with the drive transistor T1diode-connected, the initialization voltage also is applied at the first(top) plate of the storage capacitor Cst.

Further during the first initialization phase, a reference voltage thatis applied at the second (bottom) plate of the storage capacitor Cst,VREF, is changed to a mid reference level, VREF_M, as part of theinitialization. Generally as used herein, the input reference voltageVREF can be set to three different values, referred to herein a first orhigh value, a second or mid value, and a third or low value asindicative of a relative voltage of VREF during different phases of use.At the beginning of the first initialization phase as shown in FIG. 2,the VREF signal level is changed from the high value to the mid value. Adata voltage VDAT applied to transistor T4 (which currently is in theoff state) is also raised to a fixed voltage level, denoted asVDAT_(REF). In exemplary embodiments, VDAT_(REF) is set as higher than ahighest data voltage that can be applied during a subsequent dataprogramming phase, denoted VDAT_(Highest), which is a data voltage valuecorresponding to lowest greyscale value but low enough to ensure theOLED is not turned on during a subsequent threshold voltage compensationphase.

Also for effective threshold voltage compensation of the drivetransistor T1, the applied VDAT voltage should be higher thanV_(TH)+V_(INIT_L)+ΔV:VDAT>|V _(TH) |+V _(INIT_L) ΔV

More particularly, ΔV is a voltage that is large enough to generate ahigh initial current to charge the storage capacitor within onehorizontal time. The value of ΔV will depend on the properties of thetransistors. For example, ΔV would be greater than three volts for oneof low-temperature polycrystalline silicon thin film transistorprocesses.

These voltage relationships are illustrated by the following:|V _(TH) |+V _(INIT_L) +ΔV<VDAT_(Highest) <VDAT_(REF) <|V _(TH) _(OLED)|+|V _(TH) |+V _(ELVSS)wherein V_(TH) is the threshold voltage of the drive transistor T1 andV_(TH) _(OLED) is the threshold voltage of the OLED. As defined herein,a threshold voltage of the OLED is the maximum voltage differencebetween the anode and cathode of the OLED for which the output luminanceof the OLED is less than 1.0%, and preferably less and 0.1%, of themaximum luminance of the OLED during any emission phase of the circuit.

At the end of the initialization phase, the SCAN1 signal level ischanged from a low voltage value to a high voltage value, causingtransistor T3 to be turned off. Consequently, the node N1 becomesfloating.

The TFT circuit 10 next is operable in a threshold voltage compensationphase, during which any variation in the threshold voltage of the drivetransistor T1 is compensated. During such phase, a SCAN signal level ischanged from a high voltage value to a low voltage value, causingtransistor T4 to be turned on. Consequently, the data voltage,VDAT_(REF), described above is applied to the source of the drivetransistor T1. From the initialization phase, the drive transistor T1still is diode-connected through transistor T2. Accordingly, thediode-connected node N1 of the drive transistor T1 is pulled up untilthe voltage between the gate and source of the drive transistor, V_(GS),is equal to the threshold voltage V_(TH). By such operation, at the endof this compensation phase the voltage at node N1 is approximately equalto VDAT_(REF)+V_(TH).

The TFT circuit 10 next is operable in a data programming phase, duringwhich a current data voltage is programmed for driving the OLED duringthe later emission phase. During the programming phase, the data voltageVDAT is changed from the reference date voltage VDAT_(REF) to a currentdata voltage value corresponding to the required OLED current in thesubsequent emission phase, VDAT, i.e. the required greyscale value forthe pixel in the current frame. The reference voltage VREF at the secondplate of the storage capacitor Cst is changed from the mid level down toa low level, VREF_L, which pulls the voltage of the gate of the drivetransistor, V_(N2), to:V _(INIT_L) ≤V _(N2) <VDAT_(lowest) +V _(TH)wherein VDAT_(lowest) is the data voltage corresponding to the highestgreyscale value for light emission from the OLED. This prevents anylight emission during the programming phase.

At the end of the programming phase, therefore, the voltage at node N1is approximately equal to VDAT+V_(TH). Further at the end of theprogramming phase, the SCAN signal level is changed from a low voltagevalue to a high voltage value, causing transistor T4 to be turned off.Consequently, the source of the drive transistor T1 becomes floating.The SCAN2 signal level is changed from a low voltage value to a highvoltage value, causing transistor T2 to be turned off. Consequently, thegate of driving transistor is disconnected from the drain, meaning thedrive transistor T1 becomes no longer diode-connected.

As described previously, for effective threshold voltage compensation ofthe drive transistor T1, the VDAT voltage should be:VDAT>|V _(TH) |+V _(INIT_L) +ΔVin which ΔV>3V. Furthermore, to ensure no light emission during thecompensation phase:VDAT<|V _(TH) _(OLED) |+|V _(TH) +V _(ELVSS)with V_(TH) being the threshold voltage of the drive transistor T1, andV_(TH) _(OLED) being the OLED threshold voltage as defined above.Generally, the current through the OLED is less than 100 pA. Ordinarily,to satisfy both of these requirements, the VDAT range would need to besmall, which is undesirable because control of the greyscale level foremission becomes difficult. The control of the pixel circuits describedin this disclosure overcomes this limitation by splitting the thresholdvoltage compensation phase and data programming phase into two differentphases. In this manner, the two above requirements can be separately metin the different phases, and both effective threshold voltagecompensation and an optimally wide VDAT operational range are achieved.

The TFT circuit 10 next is operable in an emission phase during whichthe OLED is capable of emitting light. At the beginning of this phase,an emission second initialization phase may be performed.

In an emission second initialization phase, the voltage across the OLEDis initialized or reset in preparation for the remaining emission phase.This phase further enhances the accuracy of the threshold voltagecompensation of the drive transistor, particularly for low emissioncurrents of the OLED.

At the end the data programming phase above, the voltage at the drain ofthe drive transistor T1 is approximately equal to VDAT+V_(TH).Therefore, this voltage depends on the threshold voltage of the drivetransistor T1. If the programmed current, i.e. the current that thedrive transistor T1 will pass during the subsequent emission phase, islow, it could take some time to adjust the voltage at the drain of thedrive transistor T1 to the correct level, and consequently the V_(ds)(the voltage between the drain and source) across the drive transistorT1 could be different for the same programmed data. The secondaryeffects of V_(ds) could cause the OLED current to be different beforethe drain voltage reaches the correct level. The emission secondinitialization phase helps to mitigate this problem. In particular, thisphase improves the threshold voltage compensation accuracy particularlyin circumstances in which the required OLED current in the subsequentemission phase is low.

During this emission second initialization phase, the initializationvoltage INIT is changed from the first low voltage value, INIT_L, to asecond high voltage value, INIT_H. The SCAN1 signal level is changedfrom a high voltage value to low voltage value, causing transistor T3 tobe turned on. Consequently, node N1, which connects the drain of thedrive transistor T1 and the anode of the OLED, is connected to INIT_H.By re-initializing the node N1, the starting voltages are the same leveleven when the threshold voltage of the drive transistor T1 varies.Having re-initialized node N1, at the end of this phase the SCAN1 signallevel is changed from a low value to a high value, causing transistor T3to be turned off.

Referring again to the emission phase, VREF is boosted back to the highvoltage level, VREF_H, which boosts the gate voltage of the drivetransistor T1 to the desired operational voltage range:V _(N2) =VDAT+V _(TH) +ΔV _(REF)where ΔV_(REF)=VREF_H−VREF_L.

The operational range is the gate voltage range of the drive transistorwith which the drive transistor can control the amount of current to thelight-emitting device from the lowest current to the highest current forthe pixel circuit. For example, the current range is from 10 pA to 100nA in some applications.

The EMI signal level is changed from high to low, causing transistor T5to be turned on. The drive transistor T1 thus is connected to the secondpower supply, VDD, and conducts I_(OLED) current to the OLED as follows:

$I_{OLED} = {{\frac{\beta}{2}\left( {{VDAT} + V_{TH} + {\Delta\;{VREF}} - {VDD} - V_{TH}} \right)^{2}} = {\frac{\beta}{2}\left( {V_{DAT} + {\Delta\;{VREF}} - {VDD}} \right)^{2}}}$$\mspace{79mu}{{{{where}\mspace{14mu}\beta} = {\mu_{n} \cdot C_{ox} \cdot \frac{W}{L}}},}$

-   C_(ox) is the capacitance of the drive transistor gate oxide;-   W is the width of the drive transistor channel;-   L is the length of the drive transistor channel (i.e. distance    between source and drain);-   μ_(n) is the carrier mobility of the drive transistor.

Accordingly, the current to the OLED does not depend on the thresholdvoltage of the drive transistor T1, and hence the current to the OLEDdevice I_(OLED) is not affected by threshold voltage variations of thedrive transistor. In this manner, variation in the threshold voltage ofthe drive transistor has been compensated.

FIG. 3 is a drawing depicting a second circuit configuration 20 inaccordance with embodiments of the present invention, and FIG. 4 is atiming diagram associated with the circuit configuration of FIG. 3. Inthis example, similarly as in the previous embodiments, the circuit 20is configured as a TFT circuit that includes multiple transistors, whichin this embodiment are p-type transistors (T1-T6), and Cst is acapacitor. The circuit elements drive a light-emitting device, such asfor example an OLED. The light-emitting device (OLED) has an associatedinternal capacitance, which is represented in the circuit diagram asC_(oled). T1 is a drive transistor that is an analogue TFT, and T2-T6are digital switch TFTs, with T6 being an additional transistor in thisembodiment. C_(oled) is the internal capacitance of the OLED device(i.e., C_(oled) is not a separate component, but is inherent to theOLED). The OLED further is connected to the first power supply ELVSS.

Compared with the previous embodiment, a fixed VDAT_INIT voltage signalis added and applied through the additional sixth transistor T6 that isconnected to the third terminal of the drive transistor T1. Applicationof VDAT_INIT is controlled by the adjacent (e.g. previous) row scansignal SCAN (n−1) (i.e., “n” denotes the current row, and thus, “n−1”denotes the previous row so SCAN(n−1) is the same signal as is suppliedas the SCAN(n) signal for the previous row n) for threshold voltagecompensation. The input VDAT provides the data voltage for each row. Inthis way, the programming speed or accuracy can be improved. Inparticular, as seen with reference to the timing diagram of FIG. 4, byemploying a signal input SCAN(n−1) from the previous row for thresholdvoltage compensation, the horizontal time is significantly reduced. Thisenhanced efficiency is traded off with the use of an additional sixthtransistor T6, which results in a larger circuit as compared to theprevious embodiment.

The pixel circuit 20 of this embodiment also is operable in a firstinitialization phase, during which memory effects from residual voltagesfrom the previous frame are essentially eliminated. During this firstinitialization phase, an EMI (n) signal level (EMI signal for thecurrent row) is set to a high voltage level, causing transistor T5 to beoff. The source of the drive transistor T1 thus is disconnected from thepower supply, VDD. Comparably as in the previous embodiments, theinitialization voltage INIT also is changed from a high referencevoltage level which is similar to a low light emission voltage, INIT_H,at the anode of the OLED, to a low reference voltage level, INIT_L,which may reverse bias the OLED but does not degrade the OLED asdetailed above.

Further during the first initialization phase, the SCAN1(n) signal level(SCAN1 signal for the current row) is changed from a high voltage levelto low voltage level, causing transistor T3 to be turned on. The lowinitialization voltage, INIT_L, is thus applied at the anode of theOLED. Any residual voltage and memory effect from the previous framewill be reset to INIT_L. The SCAN2(n) signal level also changes fromhigh to low, causing transistor T2 to be turned on. The drive transistorT1 thus becomes diode-connected through transistor T2, and thereby thegate and drain of the drive transistor T1 also are initialized to theINIT_L through T2 and T3. With the drive transistor T1 diode-connected,the initialization voltage also is applied at a first (top) plate of thestorage capacitor Cst. Similarly as in the previous embodiment, T3preferably is turned on before T2.

Further during the first initialization phase, the reference voltage atthe second (bottom) plate of the storage capacitor Cst, VREF is reducedto the mid reference level, VREF_M, as part of the initialization. Atthe end of the initialization phase, the SCAN1(n) signal level ischanged from low to high, causing transistor T3 to be turned off, whichrenders node N1 floating.

The TFT circuit 20 next is operable in a threshold voltage compensationphase, during which any variation in the threshold voltage of the drivetransistor T1 is compensated. During such phase, a SCAN(n−1) signallevel (SCAN signal from the previous row) is changed from a high voltagelevel to a low voltage level, causing transistor T6 to be turned on. Thefixed voltage, VDAT_INIT, is thus applied to the source of the drivetransistor. The diode-connected node of the drive transistor is pulledup until the V_(GS) (gate to source voltage) of the drive transistor T1is equal to the threshold voltage V_(TH). At the end of this phase, thevoltage at node N1 is VDAT_INIT+V_(TH). The voltage VDAT_INIT is set ashigher than the highest data voltage range but low enough to ensure theOLED is not turned on during threshold voltage compensation.

Similarly as described previously, for effective threshold voltagecompensation of the drive transistor T1, the applied VDAT voltage shouldbe higher than V_(TH)+V_(INIT_L)+ΔV:VDAT>|V _(TH) |+V _(INIT_L) +ΔV

Accordingly, the following relationship is satisfied:|V _(TH) +V _(INIT_L) +ΔV<VDAT_(Highest) <VDAT_INIT<|V _(TH) _(OLED)|+|V _(TH) |+V _(ELVSS)At the end of this phase, the SCAN(n−1) signal level is changed from lowto high, causing transistor T6 to be turned off.

The TFT circuit 20 next is operable in a data programming phase, duringwhich data voltage is programmed for driving the OLED during the lateremission phase. During the programming phase, the data voltage ischanged from previous row data, VDAT(n−1), to current row data, VDAT(n).The SCAN(n) signal level is changed from a high voltage level to a lowvoltage level, causing transistor T4 to be turned on. The data voltageVDAT(n) is applied at the source of the drive transistor. The referencevoltage VREF at the second (bottom) plate of the capacitor is reduced toa level, VREF_L, which pulls the gate voltage of the drive transistorT1, V_(H2), toV _(INIT_L) ≤V _(N2) <VDAT_(lowest) +V _(TH)

Accordingly, at the end of this phase, the voltage at node N1 isVDAT(n)+V_(TH). In addition, the SCAN(n) signal level is changed fromlow to high, causing transistor T4 to be turned off. The source of thedriving transistor T1 thus becomes floating. The SCAN2(n) signal levelis changed from low to high, causing transistor T2 to turned off, andthe gate of drive transistor T1 is disconnected from the drain such thatthe drive transistor T1 is no longer diode-connected through transistorT2, and the current data voltage effectively is stored by the storagecapacitor Cst.

The TFT circuit 20 next is operable in an emission phase during whichthe OLED is capable of emitting light. At the beginning of this phase,an emission second initialization phase may be performed. During theemission second initialization phase, the voltage across the OLED isinitialized or reset in preparation for the remaining emission phase.Similarly as in the previous embodiment, this phase improves thethreshold voltage compensation accuracy particularly for circumstancesof low emission current. During this emission second initializationphase, the initialization voltage INIT is changed from the first lowvoltage level, INIT_L, to the second high voltage level, INIT_H. TheSCAN1(n) signal level is changed from a high voltage level to a lowvoltage level, and transistor T3 is turned on. Node N1, which connectsthe drain of the drive transistor T1 and the anode of the OLED, is thusconnected to INIT_H.

As described above, at the end the threshold voltage compensation phase,the drain voltage of the drive transistor T1 could be different. If theprogrammed current for light emission is low, it could take a long timefor the drain voltage of the drive transistor T1 to recover. The voltageat the anode of the OLED could vary so the light emission may varydepending on the threshold voltage of the drive transistor T1. Byre-initializing the node N1, the starting voltages are the same leveleven when the threshold voltages vary.

At the end of the emission second initialization phase, the SCAN1(n)signal level is changed from low to high causing transistor T3 to beturned off. In addition, VREF is also boosted to the high voltage level,VREF_H, which boosts the gate voltage of the drive transistor T1 deviceto the desired operational voltage range:V _(N2) =VDAT+V _(TH) +ΔV _(REF)

Referring again to the emission phase, the EMI(n) signal level ischanged from a high voltage level to a low voltage level, causingtransistor T5 to be turned on. The drive transistor T1 thus is connectedto the second power supply, VDD and conducts current to the OLED asfollows:

$I_{OLED} = {{\frac{\beta}{2}\left( {{VDAT} + V_{TH} + {\Delta\;{VREF}} - {VDD} - V_{TH}} \right)^{2}} = {\frac{\beta}{2}\left( {V_{DAT} + {\Delta\;{VREF}} - {VDD}} \right)^{2}}}$$\mspace{79mu}{{{{where}\mspace{14mu}\beta} = {\mu_{n} \cdot C_{ox} \cdot \frac{W}{L}}},}$

-   C_(ox) is the capacitance of the drive transistor gate oxide;-   W is the width of the drive transistor channel;-   L is the length of the drive transistor channel (i.e. distance    between source and drain);-   μ_(n) is the carrier mobility of the drive transistor.

Accordingly, the current to the OLED does not depend on the thresholdvoltage of the drive transistor T1, and hence the current to the OLEDdevice I_(OLED) is not affected by threshold voltage variations of thedrive transistor. In this manner, variation in the threshold voltage ofthe drive transistor has been compensated.

Although embodiments described herein are circuits using p-type TFTs,those skilled in the art can apply the invention also to circuits usingn-type TFTs. Furthermore, although embodiments described herein includelight-emitting devices with an anode connected to the drive transistor,the cathode of a light-emitting device may alternatively be connected tothe drive transistor. For example, the advantages of the describedcircuit configurations may be obtained for a circuit using n-type TFTsand the cathode of the light-emitting device connected to the drivetransistor.

The described pixel circuits have advantages over conventionalconfigurations. Such circuit configurations are capable of compensatingthe threshold voltage variations with fewer transistors than inconventional configurations, with additionally removing the possiblememory effects associated with the OLED device and drive transistor fromthe previous frame. The described circuit configurations, therefore,improve the capability of a pixel to emit very little or no light andtherefore have a true black state, and over a wide range of data values.The pixel circuit configurations of the present disclosure furtherimprove the true blackness by reversely biasing the OLED during phasesprior to emission, and in particular, by further employing the emissionsecond initialization phase, accuracy of threshold voltage compensationof the drive circuit is improved especially for low OLED currents. Inaddition, the voltage range for the pixel circuit (i.e., the differencebetween the highest voltage in the pixel circuit and the lowest voltagein the pixel circuit) is reduced while still providing effectivecompensation of the threshold voltage of the drive transistor. Thereduced range for the pixel circuit enables lower power consumption fora driver of the display as compared to conventional configurations.

The advantages may be achieved using as few as only five transistors andone storage capacitor, thereby providing a smaller circuit as comparedto conventional configurations. By the addition of a sixth transistor inthe second embodiment, although the circuit size is increased, thecircuit size still is small as compared to conventional configurations,and a significantly shortened horizontal time is achieved.

The various embodiments have been described in connection with OLEDs asthe display light-emitting device. The circuit configurations, however,are not limited to any particular display technology. For example, thecircuit configurations also may also be used for micro LED displays,quantum dot LED displays, or any other device which emits light inresponse to an applied electrical bias. A micro LED, for example, is asemiconductor device containing a p-type region, an n-type region and alight emission region, for example formed on a substrate and dividedinto individual chips. A micro LED may be based on a III-nitridesemiconductor. A quantum dot LED, for example, is a device containing ahole transport layer, an electron transport layer, and a light emissionregion, wherein the light emission regions contains nanocrystallinequantum dots. The circuit configurations, described herein may beemployed for any such display technologies.

An aspect of the invention is a pixel circuit for a display device thatperforms compensation of variations in drive transistor properties in asmall configuration, with additionally removing the possible memoryeffects from the previous frame. In exemplary embodiments, the pixelcircuit includes a drive transistor configured to control an amount ofcurrent to a light-emitting device depending upon a voltage applied to agate of the drive transistor; a second transistor connected to the gateof the drive transistor and a second terminal of the drive transistorsuch that, when the second transistor is in an on state the drivetransistor becomes diode-connected such that the gate and a secondterminal of the drive transistor are connected through the secondtransistor; a light-emitting device that is connected at a first node tothe second terminal of the drive transistor and at a second node to afirst voltage supply; a third transistor that is connected between aninitialization voltage supply and the first node of the light-emittingdevice, wherein a node N1 is a connection of the second terminal of thedrive transistor, the first node of the light-emitting device, and thethird transistor; and at least one capacitor having a first plate thatis connected to the gate of the drive transistor and a second plate thatis connectable to a reference voltage supply. The pixel circuit mayinclude one or more of the following features, either individually or incombination.

In an exemplary embodiment of the pixel circuit, the pixel circuit isoperable in an initialization phase, and during the initialization phasethe second and third transistors are in an on state such that a firstinitialization voltage is connected to the first plate of the capacitor,the gate of the drive transistor, and the first node of thelight-emitting device.

In an exemplary embodiment of the pixel circuit, the firstinitialization voltage is lower than the first voltage supply.

In an exemplary embodiment of the pixel circuit, the pixel circuit isoperable in a compensation phase, and during the compensation phase areference data voltage is applied at the third terminal of the drivetransistor and the reference voltage applied at the second plate of thecapacitor has a first voltage; and the pixel circuit is operable in aprogramming phase, and during the programming phase a data voltagecorresponding to a greyscale value is applied at the third terminal ofthe drive transistor and the reference data voltage applied the secondplate of the capacitor has a second voltage.

In an exemplary embodiment of the pixel circuit, the pixel circuitfurther is operable during an emission second initialization phase,wherein during the emission second initialization phase the thirdtransistor is in an on state to connect a second initialization voltageto the node N1.

In an exemplary embodiment of the pixel circuit, the pixel circuitfurther includes a fourth transistor that is connected to a thirdterminal of the drive transistor.

In an exemplary embodiment of the pixel circuit, the pixel circuitfurther includes a fifth transistor that is connected to the thirdterminal of the drive transistor, wherein during an emission phase thefifth transistor is in an on state to connect the drive transistor to asecond voltage supply and the reference voltage has a third voltage.

In an exemplary embodiment of the pixel circuit, the pixel circuitfurther includes a sixth transistor that is connected to the thirdterminal of the drive transistor, wherein during a compensation phasethe sixth transistor is in an on state to apply a fixed voltage to thethird terminal of the drive transistor.

In an exemplary embodiment of the pixel circuit, the transistors all arep-type transistors.

In an exemplary embodiment of the pixel circuit, the first node of thelight-emitting device is an anode and the second node of the lightemitting device is a cathode.

In an exemplary embodiment of the pixel circuit, the light-emittingdevice is one of an organic light-emitting diode, a micro light-emittingdiode (LED), or a quantum dot LED.

Another aspect of the invention is a method of operating a pixel circuitfor a display device including the steps of: providing a pixel circuitaccording to any of the embodiments; and performing an initializationphase, a compensation phase, a programming phase, and an emission phase.During the initialization phase, memory effects from previous frames arereduced by performing the steps of: placing the fourth transistor in theoff state, and placing the fifth transistor in the off state todisconnect the second voltage supply from the pixel circuit; placing thethird transistor in the on state to apply a first initialization voltageto the first node of the light-emitting device; setting the referencevoltage to a first voltage; placing the second transistor in an onstate, wherein the drive transistor becomes diode-connected through thesecond transistor such that the first initialization voltage is appliedto the gate of the drive transistor and the first plate of thecapacitor; and placing the third transistor in the off state at an endof the initialization phase. During the compensation phase, a thresholdvoltage of the drive transistor is at least partially compensated by thesteps of: placing the fourth transistor in an on state and applying areference data voltage through the fourth transistor to the thirdterminal of the drive transistor, wherein the reference data voltage isset such that a voltage across the light-emitting device is below athreshold voltage of the light emitting device. During the programmingphase, a data voltage corresponding to a current greyscale value isapplied by the steps of: setting the reference voltage to a secondvoltage for programming the full data range; and changing the datavoltage supply from the reference data voltage to a current data voltagecorresponding to a required current through the light-emitting deviceduring the emission phase; placing the fourth transistor in an off stateafter applying the current data voltage; and placing the secondtransistor in an off state such that the drive transistor is no longerdiode-connected, wherein the current data voltage is stored by thestorage capacitor. The emission phase includes the steps of: setting thereference voltage to a third voltage to change the gate voltage of thedrive transistor to the operational voltage range, in which the drivetransistor controls the amount of current to the light-emitting device;placing the fifth transistor in the on state to connect the second powersupply to the drive transistor; and controlling an amount of current tothe light-emitting device depending upon a voltage at the gate of adrive transistor. The operating method may include one or more of thefollowing features, either individually or in combination.

In an exemplary embodiment of the operating method, during theinitialization phase the third transistor is placed in the on stateprior to placing the second transistor in the on state.

In an exemplary embodiment of the operating method, the firstinitialization voltage reverse biases the light-emitting device.

In an exemplary embodiment of the operating method, the referencevoltage applied to the second plate of the storage capacitor is amulti-level voltage that is different during the first initializationphase and the programming phase.

In an exemplary embodiment of the operating method, the reference datavoltage is a data value corresponding to a lowest greyscale value of thelight-emitting device.

In an exemplary embodiment of the operating method, the operating methodfurther includes, after the programming phase and at the beginning ofthe emission phase, performing an emission second initialization phaseto reset the voltage at the first node of the light-emitting device bythe steps of: placing the third transistor in an on state and applying asecond initialization voltage to the first node of the light-emittingdevice; and changing the state of the third transistor from the on stateto the off state.

In an exemplary embodiment of the operating method, the firstinitialization voltage is a lower voltage than the second initializationvoltage,

In an exemplary embodiment of the operating method, the pixel circuitfurther comprises a sixth transistor that is connected to the thirdterminal of the drive transistor, wherein during the compensation phasethe sixth transistor is in an on state to apply a fixed voltage throughthe sixth transistor to the third terminal of the drive transistor.

In an exemplary embodiment of the operating method, the light-emittingdevice is one of an organic light-emitting diode, a micro light-emittingdiode (LED), or a quantum dot LED.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, it is obvious that equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiment or embodimentsof the invention. In addition, while a particular feature of theinvention may have been described above with respect to only one or moreof several illustrated embodiments, such feature may be combined withone or more other features of the other embodiments, as may be desiredand advantageous for any given or particular application.

INDUSTRIAL APPLICABILITY

Embodiments of the present invention are applicable to many displaydevices to permit display devices of high resolution with effectivethreshold voltage compensation and true black performance. Examples ofsuch devices include televisions, mobile phones, personal digitalassistants (PDAs), tablet and laptop computers, desktop monitors,digital cameras, and like devices for which a high resolution display isdesirable.

REFERENCE SIGNS LIST

-   10—first circuit configuration-   20—second circuit configuration-   T1—drive transistor-   T2-T6—multiple switch transistors-   Cst—storage capacitor-   C_(oled)—internal capacitance of light-emitting device-   ELVSS—first power supply-   VDD—second power supply-   VDAT—data voltage supply-   VREF—reference voltage supply-   INIT—initialization voltage supply-   VDAT_INIT—initialization data voltage supply-   N1—first circuit node-   N2—second circuit node-   SCAN/EMI—control signals-   1H—one horizontal time

What is claimed is:
 1. A pixel circuit for a display device comprising:a drive transistor configured to control an amount of current to alight-emitting device depending upon a voltage applied to a gate of thedrive transistor; a second transistor connected to the gate of the drivetransistor and a second terminal of the drive transistor such that, whenthe second transistor is in an on state the drive transistor becomesdiode-connected such that the gate and a second terminal of the drivetransistor are connected through the second transistor; a light-emittingdevice that is connected at a first node to the second terminal of thedrive transistor and at a second node to a first voltage supply, whereinthe gate of the drive transistor is connected to the first node of thelight-emitting device only through the second transistor; a thirdtransistor that is connected between an initialization voltage supplyand the first node of the light-emitting device, wherein a node N1 is aconnection of the second terminal of the drive transistor, the firstnode of the light-emitting device, and the third transistor; and atleast one capacitor having a first plate that is connected to the gateof the drive transistor and a second plate that is connectable to areference voltage supply; wherein the pixel circuit is operable in aninitialization phase, and during the initialization phase the second andthird transistors are in an on state such that a first initializationvoltage is connected to the first plate of the capacitor, the gate ofthe drive transistor, and the first node of the light-emitting device.2. The pixel circuit of claim 1, wherein the first initializationvoltage is lower than the first voltage supply.
 3. A method of operatinga pixel circuit for a display device that is operable in aninitialization phase, a compensation phase, a programming phase, and anemission phase; the operating method comprising the steps of: providinga pixel circuit comprising: a drive transistor configured to control anamount of current to the light-emitting device during the emission phasedepending upon a voltage applied to a gate of the drive transistor; asecond transistor connected to the gate of the drive transistor, whereinthe drive transistor can be diode-connected between the gate and asecond terminal of the drive transistor through the second transistor; alight-emitting device that is connected at a first node to the secondterminal of the drive transistor and at a second node to a first voltagesupply; a third transistor that is connected to the first node of thelight-emitting device, a fourth transistor that is connected between athird terminal of the drive transistor and a data voltage supply; afifth transistor that is connected between the third terminal of thedrive transistor and a second voltage supply; and a capacitor having afirst plate that is connected to the gate of the drive transistor and asecond plate that is connectable to a reference voltage supply; duringthe initialization phase, reducing memory effects from previous framesby performing the steps of: placing the fourth transistor in the offstate, and placing the fifth transistor in the off state to disconnectthe second voltage supply from the pixel circuit; placing the thirdtransistor in the on state to apply a first initialization voltage tothe first node of the light-emitting device; setting the referencevoltage to a first voltage; placing the second transistor in an onstate, wherein the drive transistor becomes diode-connected through thesecond transistor such that the first initialization voltage is appliedto the gate of the drive transistor and the first plate of thecapacitor; and placing the third transistor in the off state at an endof the initialization phase; during the compensation phase, at leastpartially compensating a threshold voltage of the drive transistor bythe steps of: placing the fourth transistor in an on state and applyinga reference data voltage through the fourth transistor to the thirdterminal of the drive transistor, wherein the reference data voltage isset such that a voltage across the light-emitting device is below athreshold voltage of the light emitting device; during the programmingphase applying a data voltage corresponding to a current greyscale valueby the steps of: setting the reference voltage to a second voltage forprogramming the full data range; and changing the data voltage supplyfrom the reference data voltage to a current data voltage correspondingto a required current through the light-emitting device during theemission phase; placing the fourth transistor in an off state afterapplying the current data voltage; and placing the second transistor inan off state such that the drive transistor is no longerdiode-connected, wherein the current data voltage is stored by thestorage capacitor; and during the emission phase, performing the stepsof: setting the reference voltage to a third voltage to change the gatevoltage of the drive transistor to the operational voltage range, inwhich the drive transistor controls the amount of current to thelight-emitting device; placing the fifth transistor in the on state toconnect the second power supply to the drive transistor; and controllingan amount of current to the light-emitting device depending upon avoltage at the gate of a drive transistor.
 4. The operating method ofclaim 3, wherein during the initialization phase the third transistor isplaced in the on state prior to placing the second transistor in the onstate.
 5. The operating method of claim 3, wherein the firstinitialization voltage reverse biases the light-emitting device.
 6. Theoperating method of claim 3, wherein the reference voltage applied tothe second plate of the storage capacitor is a multi-level voltage thatis different during the first initialization phase and the programmingphase.
 7. The operating method of claim 3, wherein the reference datavoltage is a data value corresponding to a lowest greyscale value of thelight-emitting device.
 8. The operation method of claim 3, furthercomprising, after the programming phase and at the beginning of theemission phase, performing an emission second initialization phase toreset the voltage at the first node of the light-emitting device by thesteps of: placing the third transistor in an on state and applying asecond initialization voltage to the first node of the light-emittingdevice; and changing the state of the third transistor from the on stateto the off state.
 9. The operating method of claim 8, wherein the firstinitialization voltage is a lower voltage than the second initializationvoltage.
 10. The operating method of claim 3, wherein the pixel circuitfurther comprises a sixth transistor that is connected to the thirdterminal of the drive transistor, wherein during the compensation phasethe sixth transistor is in an on state to apply a fixed voltage throughthe sixth transistor to the third terminal of the drive transistor. 11.A pixel circuit for a display device comprising: a drive transistorconfigured to control an amount of current to a light-emitting devicedepending upon a voltage applied to a gate of the drive transistor; asecond transistor connected to the gate of the drive transistor and asecond terminal of the drive transistor such that, when the secondtransistor is in an on state the drive transistor becomesdiode-connected such that the gate and a second terminal of the drivetransistor are connected through the second transistor; a light-emittingdevice that is connected at a first node to the second terminal of thedrive transistor and at a second node to a first voltage supply; a thirdtransistor that is connected between an initialization voltage supplyand the first node of the light-emitting device, wherein a node N1 is aconnection of the second terminal of the drive transistor, the firstnode of the light-emitting device, and the third transistor; and atleast one capacitor having a first plate that is connected to the gateof the drive transistor and a second plate that is connectable to areference voltage supply; wherein the pixel circuit is operable in aninitialization phase, and during the initialization phase the second andthird transistors are in an on state such that a first initializationvoltage is connected to the first plate of the capacitor, the gate ofthe drive transistor, and the first node of the light-emitting device.12. The pixel circuit of claim 11, wherein the first initializationvoltage is lower than the first voltage supply.
 13. The pixel circuit ofclaim 11, wherein: the pixel circuit is operable in a compensationphase, and during the compensation phase a reference data voltage isapplied at a third terminal of the drive transistor and the referencevoltage applied at the second plate of the capacitor has a firstvoltage; and the pixel circuit is operable in a programming phase, andduring the programming phase a data voltage corresponding to a greyscalevalue is applied at the third terminal of the drive transistor and thereference voltage applied at the second plate of the capacitor has asecond voltage.
 14. The pixel circuit of claim 11, wherein the pixelcircuit further is operable during an emission second initializationphase, wherein during the emission second initialization phase the thirdtransistor is in an on state to connect a second initialization voltageto the node N1.
 15. The pixel circuit of claim 11, further comprising afourth transistor that is connected to a third terminal of the drivetransistor.
 16. The pixel circuit of claim 15, further comprising afifth transistor that is connected to the third terminal of the drivetransistor, wherein during an emission phase the fifth transistor is inan on state to connect the drive transistor to a second voltage supplyand the reference voltage has a third voltage.
 17. The pixel circuit ofclaim 16, further comprising a sixth transistor that is connected to thethird terminal of the drive transistor, wherein during a compensationphase the sixth transistor is in an on state to apply a fixed voltage tothe third terminal of the drive transistor.
 18. The pixel circuit ofclaim 11, wherein the transistors all are p-type transistors.
 19. Thepixel circuit of claim 11, wherein the first node of the light-emittingdevice is an anode and the second node of the light emitting device is acathode.
 20. The pixel circuit of claim 11, wherein the light-emittingdevice is one of an organic light-emitting diode, a micro light-emittingdiode (LED), or a quantum dot LED.